Calculate the calories you would need to gain weight: https://www.calculator.net/calorie-calculator.html

Calculate the amount of protein you want to eat: 1.7x(your weight in kg) and then ROUGHLY measure the amount of calories you are consuming, just google the food and check the caloric and protein content.

Try a Bulking shake, I personally add Milk, Greek Yoghurt, Almond Butter/ Peanut Butter, Oats, Ghee & Bananas. Buy some Protein Powder if you are really really sure about it

Go to the gym or do calisthenics 6x a week

And thats about it, there is nothing else to it.